Phosphating process



Patented Sept. 17, 1963 3,104,177 PHOSPHATING PROCESS Fred Corwin Goldsmith, Painesville, Ohio, assignor to The Lubrizol Corporation, Wicklilfe, Ohio, a corporation of Ohio N Drawing. Filed Dec. 12, 1961, Ser. No. 158,896

Claims. (Cl. 117 71) The present invention relates, as indicated, to a method for inhibiting corrosion. In a more particular sense, it relates to a convenient and economical method for in hibiting the corrosion of metal articles such as ferrous metal and galvanized ferrous metal articles.

The application :of aqueous phosphating solutions in the metal finishing and fabricating ant is both wide-spread and well-known. Such solutions provide means for forming an adherent, inorganic phosphate coating on a metal surface, which coating protects the metal substrate against corrosion and serves as an excellent base to receive topcoats of organic finishes such as paint, enamel, varnish, lacquer, alkyd resins, etc.

Although .the phosphating of metal articles has substantially increased their resistance to corrosion, the degree of protection provided has been less than desired for certain commercial applications. One major shortcoming has been noted, for example, in the automotive and household appliance industries, where phosphated metal articles are invariably provided with a siccative top-coat such as a top-coat of lacquer or enamel. It has been observed that when such a top-coat is scratched or scored during, for example, handling, forming, or assembling operations, the metal substrate becomes a focal point for corrosion and for a phenomenon commonly known as undercutting. Undercutting, or the loosening of the top-coat in areas adjacent to a scratch or score, causes a progressive flaking of the top-coat from the affected area. In severe cases, the undercutting may extend an inch or more from each side of the scratch or score, causing a loosening and subsequent flaking of the top-coat from a substantial portion, if not all, of the phosphated and top-coated metal article.

In an effort to improve the corrosion resistance of phosphated metal surfaces and thereby reduce the extent of undercutting, workers in the art have investigated the use of various aqueous solutions or rinses to seal the minute pores in the phosphate coating and passivate the metal substrate. Among the more successful rinses discovered for this purpose are dilute aqueous solutions of chromic, phosphoric, or oxalic acid as well as dilute aqueous solutions of alkali or alkaline earth metal dichromates acidified with one or more of the foregoing acids; To impart the optimum degree of protection to conventional, commercial, phosphated metal surfaces, such rinses must have a free acid value in the range from about 0.05 to about 2. 0, generally from 0.1 to 1.0. The free acid value referred to is the number ofmilliliters of 0.1 normal aqueous sodium hydroxide required to neutralize a 25 milliliter sample of the rinse in the presence of bromcresol green as an indicator. These known acid rinses have proved effective in reducing the extent of corrosion and undercutting, but have not eliminated them.

It is an object, therefore, of the present invention to provide a method for inhibiting the corrosion of metal articles.

Another object is to provide a method for eliminating the incidence of corrosion and undercutting in phosphated and top-coated metal articles.

Still another object is to provide phosphated and topcoated metal articles which are resistant to the ravages of highly corrosive atmospheres.

These and other objects of the invention are achieved by a method which comprises (A) phosphating a metal article with an aqueous phosphating solution containing as essential ingredients zinc ion, phosphate ion, nitrate ion, and a cation selected from the group consisting of lithium, beryllium, magnesium, calcium, strontium, cadmium, and barium to form thereon an integral phosphate coating, and then (B) rinsing said phosphated metal article with a dilute aqueous solution of calcium dichromate devoid of free acid.

The novel phosphating solutions essential for the purpose and practice of this invention are described in detail in co-pending US. application Ser. No. 373,449, filed August 10, 1953, by John A. Henricks. It is intended that the entire disclosure of the said Henricks application be incorporated'herein by reference. The acidic, aqueous phosphating solutions described therein preferably have a total acidity within the range from about 5 to'about 100 points and contain as essential ingredients zinc ion, phosphate ion, nitrate ion, and a cation selected from the group consisting of lithium, beryllium, magnesium, calcium, strontium, cadmium, and barium. Such phosphating solutions provide a dense, adherent, micro-crystalline or amorphous phosphate coating which shows substantially no visible crystal structure at a magnification of diameters. The points total acidity or total acid referred to above represents the number of milliliters of 0.1 normal sodium hydroxide solution required to neutralize a 10 milliliter sample of the phosphating solution in the presence of phenolphthalein as an indicator. Generally a total acidity within the range from about 5 to about 100 points, preferably from about 5 to about 50 points, is required to obtain phosphating solutions which are capable of providing commercially satisfactory coating weights and speeds.

Of the various phosphating compositions disclosed in Ser..No. 373,449, a particularly effective and useful subgroup for the purpose of the present invention includes those which have a total acidity within the range from about 5 to about 100 points and which contain as essential ingredients from about 0.1 to about 1.0 percent of zinc ion; from about 0.25 to about 2.0 percent ofphosphate ion; from about 0.25 to about 8.0 percent of nitrate ion; and from about 0.1 to about 4.0 percent of a cation selected from the group consisting of lithium, beryllium, magnesium, calcium, strontium, cadmium, and barium.

Best results from the standpoint of the excellence of the phosphate coating and economy of ingredients are obtained when the phosphating step of the present invention is carried out by means of an aqueous phosphating solution having a total acidity within the range from about 5 to about 50 points and containing as essential ingredients from about 0.1 to about 0.6 percent of zinc ion; from about 0.3 to about 1.5 percent of phosphate ion; from about 0.5 to about 6. 0 percent of nitrate ion, and from about 0.1 to about 1.5 percent of calcium ion. In certain instances, the additional presence of the ammonium ion is desirable.

In view of the extensive commercial development of the phosphating art and the many journal publications and patents describing the application of phosphating solutions, it is believed unnecessary to lengthen this specification unduly by a detailed recitation of the many ways in which the phosphating step may be accomplished. Suffice it to say'that any of the commonly used phosphating techniques such as spraying, brushing, dipping, flowcoating, or roller-coatiug may be employed, and that the temperature of the aqueous phosphating solution may vary within widelimits, erg, from room temperature to about 212 F. In general, best results are obtained when the aqueous phosphating solution is used at a temperature within the range from about 150 F. to about 210 F. If desired, however, the aqueous phosphating bath may be used at higher temperatures, e.g., 225 F., 250 F., or even 300 F., by employing superatmospheric pressures.

In the ordinary practice of phosphating a metal surface, such surface is first cleaned by physical and/or chemical means to remove any grease, dirt, or oxides, and then it is phosphated in the manner described earlier. The phosphating operation is usually carried out until the weight of the phosphate coating formed on the metal surface is at least about 25 milligrams per square foot of surface area and is preferably within the range from about 100 to about 1000 milligrams per square foot of surface area. The time required to form the phosphate coating will vary according to the temperature, the type of phosphating solution employed, the particular technique of applying the phosphating solution, and the coating weight desired. In most instances, however, the time required to produce a phosphate coating of the weight preferred fior the purpose of the present invention will be within the range from about one-quarter minute to about or minutes. Under certain'circumstances, however, such as the immersion of hot (300-700 F.) steel in a phosphating solution, the steel is phosphated almost instantaneously.

Specific examples of phosphating solutions which yield phosphate coatings suitable for the purpose of the present invention are shown in Table I (values given are the percentages by weight of the several ions in the aqueous phosphating solution).

TABLE I Phosphating Solution Points total acid The above phosphating solutions can be made conveniently by dissolving zinc dihydrogenphosphate in Water to supply the required zinc and phosphate ions, and the-n adding a nitrate of one or more metals of the group consisting of lithium, beryllium, magnesium, calcium, strontium, cadmium, and barium. Finally the acidity of the solution (i.e., the points total acid) is adjusted by the addition of small amounts of phosphoric acid or nitric acid. Alternatively, the solutions can be made by dissolving zinc nitrate in water and then adding a phosphate of at least one metal of the group consisting of lithium, beryllium, magnesium, calcium, strontium, cadmium, and barium. As indicated above, the acidity of the resulting solution may then be adjusted by the addition of small amounts of phosphoric acid or nitric acid.

The ions of the phosphating bath used in the practice of this invention may be derived from a variety of compounds and it appears to be of little consequence whether or not these ions come from different salts or acids. Regardless of the identity of the salts selected to provide the required ions, the resulting bath is effective to serve the purpose of this invention. It is necessary only that these salts or acids be used in amounts to provide the necessary concentration of the required characterizing ions. In addition to the characterizing ions present in the phosphating bath, certain supplementary ions such as chloride, bromide, ammonium, chlorate, perchlorate, nitrite, or perborate ions may also be present to control coating speed, increase the rust-inhibiting qualities of the coating, reduce sludging, etc. 7

The presence of the lithium, beryllium, magnesium,

calcium, strontium, cadmium, or barium ion serves to suppress the formation of massive, hydrated crystalline coatings and yield instead the micro-crystalline or amorphous coating required for the purpose of the present invention. The nitrate ion serves as an oxidizing agent to depolarize the metal surface and increase the coating speed of the phosphating solution. Its presence is likewise essential in the phosphating solutions employed for the purpose of the present invention.

By way of illustration, solution A in Table I was prepared by dissolving in suflicient water to make one liter of solution, 14.2 grams of Zn(N0 -6H O, 7.8 grams of commercial 75 percent H PO 4.2 grams of ZnCl 8.7 grams of NH H PO and 14.3 grams of Solution B in Table I was prepared by dissolving in suflicient water to make one liter of solution, 21.4 grams of Zn(NO -6H O, 7.2 grams of commercial 75 percent H PO 8.4 grams of NH H IO and 18.1 grams of Ca(NO -3H O.

After the article has been provided with a microcrystalline phosphate coating in the manner indicated, it is rinsed, optionally, with water and then rinsed with the dilute aqueous solution of calcium dichrornate devoid of free acid.

To serve the purposes of this invention, the aqueous calcium dichromate solution employed must be devoid of any free acid, -i.e., it must have a free acid value of zero. As shown hereinafter, the presence of any free acid in such solution prevents the attainment of an important object of this invention, namely, the elimination of corrosion and undercutting in phosphated and topcoated metal articles.

The dilute aqueous solution of calcium dichromate required herein contains from about 0.01 to about 0.4 percent, preferably from about 0.02 to about 0.1 percent, of CrO as calcium dichromate. Such solution is made conveniently by dissolving from about 0.013 part to about 0.51 part, preferably from about 0.025 part to about 0.13 part, of anhydrous calcium dichromate (OaCr o in sufficient water to yield parts of solution. If desired, the calcium dichromate can be made in situ by the reaction of a slurry of 1.72 parts of calcium hydroxide in 6.5 parts of water with 5.1 parts of CrO for about 8 hours at -120 F. to yield an aqueous concentrate containing 45 percent of calcium dichromate. This concentrate is then diluted with additional water to yield the dilute solution required for the purposes of this invention.

The rinsing of the phosphated metal article with the dilute aqueous solution of calcium dichromate may be accomplished by any one of the commonly used rinsing methods such as dipping, spraying, or flow-rinsing. The solution is generally maintained at a temperature in the range 33 to 210 F., generally 60 to F., although higher temperatures such as 225 F., 250 F., or even 300 F. may be used in conjunction with superatmospheric pressures. The duration of the rinsing operation is not critical, :a few seconds to several minutes or more being suitable.

It has been found that the phosphate coating specified for the purpose of this invention cooperates in a unique and as yet unknown manner with the aqueous calcium dichromate solution devoid of free acid to eliminate the problem of corrosion and undercutting in phosphated and top-coated metal articles. This is quite unexpected in view of test data which demonstrate that in the sealing of conventional zinc phosphate coatings, acid rinses such as dilute, acidified calcium dichromate solutions and dilute solutions of chromic acid-phosphoric acid mixtures are as effective or in some instances more effective than a calcium dichromate solution which is devoid of free acid. On the other hand, such known acid or acidified salt rinses are inefiective to eliminate undercutting'when' em ployed with the novel zinc phosphate coa-ting of this invention.

The following examples are given to set forth specific modes of carrying out the present invention. They are intended for purposes of illustration only and are not to be construed as limiting the scope of the appended claims.

EXAMPLE 1:

A large number of clean, degreased, 4*inc-h x 12-inch panels of galvanized, SAE 1020, -gauge cold-rolled steel were spray-phosphated for 70 seconds at 160165 F. with an aqueous solution containing 0.15 percent zinc ion, 0.54 percent phosphate ion, 1.40 percent nitrate ion, and 0.39 percent calcium ion, and having a total acidity of 12 points. The panels were rinsed with water 'atroom temperature and grouped into eight sets of 5 panels each.

The 8 sets of panels were then spray-rinsed, respectively, with 8 different aqueous rinses (identified below) for 30 seconds at 70 F.

Aqueous rinses of the invention (devoid of free acid)- A-l: 0.025 percent CrO as calcium dichromate A-2: 0.1 percent OrO as calcium dichromate Known aqueous acid rinses B1: 0.025 percent OrO B-2: 0.1 percent CrO C-1z 0.025 percent CrO as a mixture of 2.6 parts of CrO and 1 part of calcium dichromate, plus 0.015 percent H PO C-2: 0.1 percent CrO as a mixture of 2.6 parts of CrO and 1 part of calcium dichromate, plus 0.06 percent H PO D1: 0.025 percent CrO plus 0.025 percent H PO D-2: 0.1 percent CrO' plus 0.1 percent H PO Thereafter, each set of panels was dried, spray-coated with a good, commercial, White baking-enamel, and then baked for 20 minutes at 320 F. The phosphated and enameled panels were subjected to a salt fog corrosion test to determine their resistance to undercutting.

The apparatus used for this test is described in ASTM procedure B117-57T. It consists of a chamber in which a mist or fog of 5 percent aqueous sodium chloride is maintained in contact with the test panels for a predetermined time at 95i2 F. In the instant series of tests, the coating on each panel was pierced with a pointed instrument to yield a vertical scribe or score beginning one inch from the top of the panel and ending one inch from the bottom thereof, and then the panels were placed in the chamber for 120- hours.

After removal from the chamber, the panels were inspected and rated for undercutting. The undercut rating is the average loss of enamel from each side of the scribe in the S-panel set expressed as a value which represents the number of thirty-seconds of an inchv of such loss.

The results obtained in this test are shown in Table II (the rinses are arranged in groups of like concentration for ease of comparison).

In each concentration group, the aqueous rinses of this invention (A-1 and A2) were found to be superior to known acid or acidified salt rinses. Moreover, it should 6 be noted that rinse A-2 completely eliminated undercut? ting. a

EXAMPLE 2 In a manner like that described in Example 1, eight 5- panel sets of clean, degreased, bare steel panels (not' galvanized) were phosphated, water-rinsed, and then sprayrinsed, respectively, with 8 different aqueous rinses (identified below) for 30 seconds at 70 F.

Aqueous rinses of the invention (devoid of free acid)- A-l: 0.025 percent OrO as calcium dichroma-te A-3: 0.075 percent CrO as calcium dichrom'ate The rinsed panels were prepared for the salt fog corrosion test in the same manner described in Example 1. The test results are shown in Table III (the rinses are again arranged in groups of like concentration for ease of comparison).

TABLE III Aqueous rinse: Undercut rating A-1 1.0

B-1 2.6 Cl 2.8 D-l 3.3

A-3 0.0 B-3 1.0 C-3 0.6 D-3 2.5

Once again, the aqueous rinses of this invention (A-1 and A-3) were found to be superior to known 'acid or Iacidi-fied salt rinses. It will be noted that rinse A-3 completely eliminated undercutting.

EXAMPLE 3 This experiment was carried out to determine if the aqueous icalcium dichromate rinse used in the method of this invention is also eifective in eliminating undercutting of a metal article which has been phosphated with a common, commercial zinc phosphate bath devoid of calcium ion and then enameled in the usual manner. The phosphating bath employed contained 0.15 percent zinc ion, 0.54 percent phosphate ion, 0.2 percent nitrate ion, 0.017 percent nitrite ion, and 0.08 percent sodium ion. Except for the use of this phosphating bath, the procedural steps and materials were the same as those set forth in Example 1.

The test results are shown in Table IV below.

These test results indicate that the aqueous calcium dichromate rinse essential for the method of this invention is not superior to known acid or acidified salt rinses for the improvement of ordinary, commercial zinc phosphate coatings. .Used in this manner, it is not efiective in eliminating undercutting. Moreover, these test re- 'sults emphasize the unexpected showing of the test results set forth in Tables 11 and III, wherein the cooperation between the novel phosphate coating and the aqueous calcium dichromate solution devoid of free acid is clearly seen to eliminate undercutting.

What is claimed is:

1. A method for inhibiting the corrosion of a metal article selected from the class consisting of ferrous and galvanized metal articles which comprises (A) phosphating said metal article with an aqueous phosphating solution having a total acidity within the range from about to about 100 points and containing as essential ingredients from about 0.1 to about 1.0 percent of zinc ion; from about 0.25 to about 2.0 percent of phosphate ion; from about 0.25 to about 8.0 percent of nitrate ion; and from about 0.1 to about 4.0 percent of a cation selected from the group consisting of lithium, beryllium, magnesium, calcium, strontium, cadmium, and barium to form thereon an integral phosphate coating of at least about 25 milligrams per square foot of surface area; and then (B) rinsing said phosphated metal article with an aqueous solution of calcium dichromate devoid of free acid and containing from about 0.01 to about 0.4 percent CrO as calcium dichromate.

2. A galvanized metal article the metal surface of which has been inhibited against corrosion in accordance with the method of claim 1.

3. A ferrous metal article the metal surface of which 8 has been inhibited against corrosion in accordance with the method of claim 1.

4. A metal article according to claim 3 which has been provided with a siccative top-coat.

5. A method for inhibiting the corrosion of a ferrous metal article which comprises (A) phosphating said ferrous metal article with an aqueous phosphating solution having a total acidity within the range from about 5 to about points and containing as essential ingredients from about 0.1 to about 0.6 percent of zinc ion; from about 0.3 to about 1.5 percent of phosphate ion; from about 0.5 to about 6.0 percent of nitrate ion; and from about 0.1 to about 1.5 percent of calcium cation to form thereon an integral phosphate coating of from about to about 1000 milligrams per square foot of surface area; and then (B) rinsing said phosphatedferrous metal article with an aqueous solution of calcium dichromate devoid of free acid and containing from about 0.02 to about 0.1 percent CrO as calcium dichromate.

References Cited in the file of this patent UNITED STATES PATENTS 2,500,915 Tanner Mar. 14, 1950 2,634,225 Benzing Apr. 7, 1953 2,882,189 Russell et a1. Apr. 14, 1959 2,884,351 Cavanagh et a1. Apr. 28, 1959 3,015,593 Jayne Jan. 2, 1962 

1. A METHOD FOR INHIBITING THE CORROSION OF A METAL ARTICLE SELECTED FROM THE CLASS CONSISTING OF FERROUS AND GALVANIZED METAL ARTICLES WHICH COMPRISES (A) PHOSPHATING SAID METAL ARTICLE WITH AN AQUEOUS PHOSPHATING SOLUTION HAVING A TOTAL ACIDITY WITHIN THE RANGE FROM ABOUT 5 TO ABOUT 100 POINTS AND CONTAINING AS ESSENTIAL INGREDIENTS FROM ABOUT 0.1 TO ABOUT 1.0 PERCENT OF ZINC ION; FROM ABOUT 0.25 TO ABOUT 2.0 PERCENT OF PHOSPHATE ION; FROM ABOUT 0.25 TO ABOUT 8.0 PERCENT OF NITRATE ION; AND FROM ABOUT 0.1 TO ABOUT 4.0 PERCENT OF A CATION SELECTED FROM THE GROUP CONSISTING OF LITHIUM, BERYLLIUM, MAGNESIUM, CALIUM, STRONTIUM, CADMIUM, AND BARIUM TO FORM THEREON AN INTEGRAL PHOSPHATE COATING OF AT LEAST ABOUT 25 MILLIGRAMS PER SQUARE FOOT OF SURRFACE AREA; AND THEN (B) RINSING SAID PHOSPHATED METAL ARTICLE WITH AN AQUEOUS SOLTION OF CALCIUM DICHROMATE DEVOID OF FREE ACID AND CONTAINING FROM ABOUT 0.01 TO ABOUT 0.4 PERCENT CRO3 AS CALCIUM DICHROMATE. 